1. Field of the Invention
The present invention generally relates to a turbo charging system for a diesel engine, and more particularly to such a turbo charging system that can improve both engine performance and fuel consumption rate with a simple construction and control.
2. Description of the Related Art
Diesel engines are suited for a turbo charger since unlike gasoline engines there is no need to adjust an amount of intake air for engine performance (output) control. A typical structure of a turbo charger system for a diesel engine is illustrated in FIG. 5 of the accompanying drawings. This is a one-stage turbo charger.
As illustrated, this turbo charger system 38 includes a compressor 44 provided on an intake air passage 42 of a diesel engine 40, a turbine 48 provided on an exhaust gas passage 46, and a rotating shaft 50 connecting the compressor 44 with the turbine 48. As the turbine 48 is rotated by an exhaust gas flowing in the exhaust gas line 46, its rotation is transmitted to the compressor 44 via the shaft 50. The compressor 44 then pressurizes the intake air and feeds it to the engine 40. An after cooler 52 is optionally provided between the compressor 44 and engine 40.
If engine revolution speed rises and exhaust gas flow rate (mass flow) correspondingly increases, a rotational speed of the turbine 48 rises and simultaneously the compressor 44 rotates at an increased speed. As a result, a supercharging pressure to the engine 40 may become excessively high. In order to avoid it, a bypass line 54 is provided on the exhaust gas line 46 over the turbine 48, and a bypass valve 56 is provided on the bypass line 54 for causing part of the exhaust gas to bypass the turbine 48 when the supercharging pressure exceeds a predetermined value. By opening the bypass valve 56, the rotational speed of the turbine 48 can be suppressed, i.e., the rotational speed of the compressor 44 can be suppressed. Consequently, the supercharging pressure is controlled.
Referring now to FIG. 6 of the accompanying drawings, illustrated is a performance map of the compressor 44 of the turbo charger system 38 shown in FIG. 5, together with an engine performance curve (operation curve) when running under a full load condition. The curve 60 indicates a surge limitation, the curve 62 a maximum rotation limit, the point 64 a maximum torque of the engine 40, the point 66 a maximum output of the engine 40, the point 68 a maximum efficiency of the compressor 44, and the multi-circle 70 iso-efficiency curves of the compressor 44. As understood from this diagram, when the engine 40 is running under the full load condition, the bypass valve 56 is closed until the engine demonstrates the maximum torque 64 (i.e., until the engine revolution speed reaches a corresponding value), and if the engine revolution speed exceeds that value, the bypass valve 56 is gradually opened to control the supercharging pressure.
The vertical axis of the graph shown in FIG. 6 indicates a pressure ratio, and the horizontal axis indicates a corrected mass flow, which are given by the following equations respectively:
Pressure ratio=total pressure at compressor outlet/total pressure at compressor inlet PA1 Corrected mass flow=(measured mass flow.times.(inlet temperature/reference temperature).sup.0.5)/(inlet pressure/reference pressure)
where the reference temperature=20.degree. C. PA2 the reference pressure=atmospheric pressure
(reference value for correction), and PA3 (reference value for correction).
When the engine output and torque should be raised in the above described turbo charger system 38, the turbine 48 may be tuned so that the turbine rotational speed is raised relative to the same exhaust gas flow rate (i.e., same engine revolution speed) and the compressor rotational speed is raised as well. This shifts the operation curve upwards in the graph of FIG. 6 so that it passes a high pressure ratio area. However, as the pressure ratio is raised, the efficiency drops, i.e., the high pressure ratio area is a low efficiency area. Thus, raising the operation curve into the high pressure ratio area results in deterioration of fuel consumption ratio.
Specifically, if the pressure ratio is raised and a low compressor efficiency area is used as mentioned above, the turbine 48 must perform more work (torque of the rotating shaft 50) in order to raise the intake air pressure (supercharging pressure). This brings about exit clogging of the exhaust gas, which raises the exhaust gas pressure. As a result, as shown in FIG. 8, the intake air pressure does not exceed the exhaust gas pressure, and pumping loss occurs as indicated by the shaded area. This deteriorates the fuel consumption rate. Here, the intake air pressure is an air pressure downstream of the compressor 44, and the exhaust gas pressure is an air pressure upstream of the turbine 48.
Even if the engine output and torque are suppressed and a low compressor pressure ratio area is used, the operation curve inevitably passes the low efficiency area because the centrifugal compressor 44 has a high efficiency area in a narrow flow rate range due to its structure whereas the automobile engine 40 is operated in a wide rotation speed range and the exhaust gas has a wide flow rate range. Thus, for the reason mentioned earlier, there is only a limited range of good fuel consumption rate, and it is difficult to improve the fuel consumption rate as a whole.
There is also known a variable displacement turbo charger system which aims to improve the fuel consumption rate. By changing a turbine displacement (capacity) in accordance with change of the exhaust gas flow rate, this turbo charger system intends to adjust the turbine rotational speed such that the compressor is used in a high efficiency area as much as possible. However, since the turbine displacement is variable, throttle loss occurs and exhaust gas pressure increases. Further, the variable range itself is limited. Accordingly, great improvement cannot be expected on the fuel consumption rate. In addition, a variable displacement turbine requires a complicated mechanism. This raises a manufacturing cost.
Another type of conventional turbo charger system includes two turbo chargers having different characteristics. In this system, two compressors are provided in series on intake air line of an engine and two turbines are provided on an exhaust gas line. This is a two-stage turbo charger system.) Each of the compressors and turbines has a bypass line with a bypass valve, and these bypass valves are closed and opened according to the engine running condition such that the two turbo chargers are selectively operated. In this arrangement, when one of the turbo chargers is driven, the other turbo charger is fundamentally deactivated (full switching type). Accordingly, timing control for switching is difficult, i.e., it requires complicated control.